Electrothermal printing unit

ABSTRACT

An electrothermal printing unit for writing dots matrix characters on a thermosensitive paper comprises a print head carrying a plurality of thermo-activable printing elements aligned along a printing line of the recording medium. 
     The head is reciprocated in a direction parallel to the printing line of the recording medium. The recording medium is moved incrementally perpendicular to the direction of a line of printing in synchronization with the movement of the head and control means are provided for selectively activating the printing elements at a succession of positions of the head in each of a succession of strokes thereof in an arrangement such that each element print all the points of at least one character of the line of characters during the succession of strokes.

BACKGROUND OF THE INVENTION

The present invention relates to an electrothermal printing unit of animproved kind for impact-less typing of alphanumerical characters, inaccordance with point matrices, on to a heat-sensitive recording medium.In this specification "row" refers to a row of points of a row andcolumn matrix in contrast to "line" which refers to a line ofcharacters, a row being parallel to a line.

For efficient utilization of the speed of modern systems of electronicinformation handling it is essential to have available very high-speedperipheral printing units, and it is also desirable that the printingunits should not be too numerous, since they are normally located nearthe user, in offices and other working places. The number of unitsrequired depends on the output of each unit.

The use of printing equipment with ballistic impact of acharacter-bearing member against an inked ribbon in contact with paperinvolves speed limitations intrinsic in the mechanical operation andnoise levels which often reach the limits of toleration, but theintroduction of impactless printing equipment removes these drawbacks.

Impact-less printing devices are known which make use of theelectrothermal typing method. For example, U.S. Pat. No. 3,839,630 toE.E. Olander et al, relates to an electrothermal line printer, i.e.designed to type simultaneously a complete line of characters by meansof point matrices, on to a heat-sensitive recording medium.

It comprises a printing head which carries, aligned in parallel to theprinting line, as many thermally-activable printing elements as thereare characters in a line of print, multiplied by the number of pointsconstituting a row of each matrix. For example, if the matrix is of thekind having 7 rows × 5 columns and with a line of print having 16characters, the number of elements required is 16 × 5 = 80 elements.

The head, during elementary printing act, carries out the typing of arow of matrix points for all the characters in a line of print. Completeprinting of a row of characters therefore requires as many elementaryprinting acts as there are points in a column of each matrix. During theelementary printing acts, the recording medium is fixed relative to thehead, and carries out an interlinear elementary movement equal to thedistance between two rows of the matrix after each elementary printingact.

The selection and control circuits must therefore be designed to controla number of printing elements equal to a multiple of the number ofcharacters in a line of print. The printing speed is obviously high andmechanical control is concerned solely with interlinear movement of therecording medium. But the complexity and the number of the electroniccontrol circuits for this kind of printing unit leads to seriouselectrical connection problems and above all to a very high constructioncost which is not always justified by increased performance in terms ofprinting speed.

These drawbacks are removed by the unit according to the invention,which makes it possible to obtain a speed performance comparable withthat of an electrothermal line printer of the kind described above, butusing a much smaller number of theremoelements and an elctronic controlcircuit which is consequently much smaller.

SUMMARY OF THE INVENTION

According to the present invention there is provided an electrothermalprinting unit for impact-less printing of characters in accordance witha matrix of points arranged in rows, comprising means for supporting aheat-sensitive recording medium and for moving the medium incrementallyperpendicular to the direction of a line of print for printingsuccessive rows of points, a printing head carrying a plurality ofthermo-activable printing elements aligned in the said direction, meansfor reciprocating the head in the said direction in synchronism with themovements of the recording medium, means for signalling the position ofthe head, means for storing codes designating a line of characters, andcontrol means responsive to the signalling means and the storing meansto selectively activate the printing elements at a succession ofpositions of the head in each of a succession of strokes thereof in anarrangement such that each element prints all the points of at least onecharacter of the line of characters during the succession of strokes.

The invention will be described in more detail, by way of example, withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a printing unit embodying the invention;

FIG. 2 is a plan view of a typing head for the printing unit;

FIG. 3 shows a detail of FIG. 2;

FIG. 4 is a section on the line IV--IV of FIG. 3;

FIG. 5 is a block diagram of the electronic circuit for controlling theprinting unit;

FIG. 6 is a time diagram of some logical signals generated by thecontrol unit of FIG. 5;

FIG. 7, is a time diagram of some logical signals generated by thecontrol unit of FIG. 5 with a time scale enlarged compared with FIG. 5;and

FIG. 8 shows a matrix format of points for typing characters with theprinting unit.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 there is shown a preferred embodiment of an electrothermalprinter according to the invention. This comprises an electrothermalprinting head 2 (see also FIG. 2) in which printing elements 33 arepositioned on the face 3 in contact with a heat-sensitive sheet 4 alongits typing line. The heat-sensitive recording sheet 4 in the zone facingthe head is partly wrapped on a platen 7 keyed on the shaft 8. The headis fixed on a slider 10 running along a guide 11 extending across thewidth of the sheet 4 parallel to the typing line and fixed, at oppositeends, to the frame of the machine, indicated generally by 13. Atransport system 15 imparts a reciprocatory oscillating movement alongthe guide 11.

A pawl system 17 controlled by a cam 18 gives the shaft 8 anintermittent stepwise rotary movement, so that the sheet 4 isincrementally advanced by the roller 7 in the direction indicated by thearrow 19.

A sensing device 22 is designed to signal the position of the slider 10and to furnish electrical synchronising impulses to an electronic device30 (FIG. 5) for controlling the typing operations of the printer byselecting and timing the thermal excitation of the typing elements 33 ofthe head 2.

Another sensing device 27 sends a synchronizing impulse to the controldevice 30 for each complete rotation of the cam 18. In more detail, ifwe take as an example a printer designed to form lines of print composedof 16 alphanumerical characters according to the 7 × 5 point matrix (7rows × 5 columns), the printing head 2 (FIG. 2) is composed of arectangular plate 31 made of insulating material on which are depositedparallel to its smaller side and regularly spaced along a major side 32of the same, sixteen pairs of conductors 35 and 36, having terminalportions 35a and 36a (FIGS. 3 and 4) near the side 12, opposite eachother and of equal width.

On the portions 35a and 36a there are provided sixteen resistor deposits33 which constitute the printing elements. The part 33a of the printingelements (for the resistor deposits) 33 comprised between the facingportions 35a and 36a of the conductors 35 and 36 is the only part thatcan be electrically, and hence thermally, activable, since the otherparts of the elements 33 are short-circuited by the underlyingconductors 35a and 36a. The part 33a is therefore rectangular in formwith height A equal to the width of the ends 35a and 36a, this heightcorresponding to the height of the printed points intended to beobtained on the sheet 4 (e.g. A = 200μ) and with a width B, equal to thedistance between the facing ends 35a and 36a, much smaller than thewidth of the said printed points (e.g. B = 50μ). In what follows weshall always refer to the thermally activable part 33a by the name"thermo-element" 33a. The distance between two adjacent thermo-elements33a is equal to the typing step, i.e. the pitch between correspondingcolumns of points belonging to adjacent characters of a given line ofprint.

The conductors 36 are connected to a common return conductor 37. Boththe conductors 35 and the conductor 37 have in addition terminalportions 35b and 37b regularly spaced on the side of the head oppositethe side 32, and are designed to connect with spring contacts 38supported by the slider 10. By means of wire connections not shown inFIG. 1, the contacts 38 are connected with the control unit 30.

The techniques, the materials of construction of the head 2, and alsothe advantages obtained by this type of head in typing "on the fly" onthe recording medium, have been described in the copending U.S. pat.appln. Ser. No. 512,564 filed Oct. 7, 1974 assigned to the same assigneeof the present invention which particularly describes a thermal printhead having rectangular printing elements which produce for the combinedaction of their movement and the activation of the elementssubstantially square matrix dots and therefore will not be describedherein.

The head 2 is fixed, for example by an adhesive, to a support block 9which is fixed by screws 39 to the slider 10. As already stated, theslider is mounted to run on the guide 11 and it can also rotate aroundit. A spring 40 stretched between an arm 42 of the slider 10 and theframe 13 tends to turn the slider 10 counterclockwise (in FIG. 1), thusmaking the part of the head 2 next the edge 32 and carrying the alignedresistor elements 33, rest on the typing line of the sheet 4.

The transport system 15 of the slider 10 is composed of a shaft 44 onwhich is keyed a pulley 45 which is driven by a pulley 46 keyed on ashaft 47 of a motor 48, through a belt 49 which couples the two pulleys.One the end 50 of the shaft 44 there is fixed, eccentrically withreference to the axis of the same, a peg 51 on which is pivoted one endof a bar 52 whose other end is pivoted on an arm 53 of the slider 10.

The shaft 44, the peg 51 and the bar 52 constitute a system fortransmission of motion of the crankshaft type, by which constant rotarymotion imparted to the shaft 44 of the motor 48 is transformed intoperiodic reciprocating motion of the slider along the guide 11, theradial distance of the peg 51 from the axis of the shaft 44 and thelength of the bar 52 being calculated so as to give an amplitude ofoscillation of the slider 10, and hence of the head 2, little greaterthan the width of the characters that are to be obtained on a printingline (e.g. 3 mm).

As already stated, the advancing movement of the sheet 4 is effected bymeans of the pawl system 17, governed by the cam 18, keyed on a shaft 55which is rotated by the motor 48 through a worm 56 and worm wheel 57.The pawl system 17 comprises a saw-toothed wheel 60 keyed on the shaft 8and which cooperates with a peg 61 fixed to one end of a lever 62 whoseother end is pivoted on an arm 63 of a lever 64 pivoted on the frame ofthe machine 13. To another arm 65 of the lever 64 there is fixed a peg66, cooperating with the external profile of the cam 18 under the actionof a spring 67 stretched between the arm 63 of the lever 64 and a fixedpoint of the frame 13. A spring 68 is stretched between the lever 62 andthe arm 63 of the lever 64. The external profile of the cam 18,extending through 360°, comprises seven arcs of circumference 70, 71,72, 73, 74, 75, 76, with their centres on the axis 55, of equal angularamplitudes, and with increasing radii, and a lobe 77 extending throughan angular amplitude equal to the above. The arcs 70 to 76 are connectedby ramps 70a to 75a which provide a constant radial increment betweenadjacent arcs.

The lobe 77 is composed of a ramp 78 determined by a radial incrementequal to a multiple (e.g. 3-fold) of the radial increments between thepreceding arcs, of a constant-radius track 79, and a decreasing-radiustrack 80 which joins the track 79 to the arc 70.

A positioner 81, composed of a lever 83 with one end having its fulcrumon the frame of the machine 13 and having at the other end a pivot 84 onwhich is pivoted a roller 85, cooperates, under the action of a spring86 stretched between the lever 83 and a fixed point on the frame 13, bymeans of a roller 85 with the teeth of the saw-toothed wheel 60 in orderto prevent the latter from rotating clockwise.

The pawl system 17 has, in the working phase, the following method offunctioning, assuming an initial position corresponding to thatindicated in FIG. 1, i.e. with the peg 66 at the beginning of the arc70.

When rotation of the cam 18 counterclockwise brings the ramp 70a intocollaboration with the peg 65, the lever 64 turns counterclockwisethrough a certain angle against the action of the spring 67.

The lever 62 is displaced upwards, being pulled by the spring 68 in sucha way that the peg 61 rests in the gap between two teetch 60a, 60b thusmaking the wheel 60 and the platen 7 turn through a small angle andthereby producing an advance of the sheet by an elementary increment,equal to the distance between two adjacent rows of the matrix format.The same thing is repeated during engagement of the peg 66 with each ofthe other ramps 71a to 75a, while during engagement of the peg with thearcs 70 to 76 there is no variation in the position of the lever 64 andhence none also of the wheel 60.

Continuing the rotation of the cam 18, when the peg 66 engages with theramp 78 which, as already stated, comprises a radial increment which isa multiple of those preceding, the lever 64 carries out a furtherrotation in the counterclockwise direction making the wheel 60 turnfurther counterclockwise by an angle equal to a multiple (3-fold in theexample considered) of the preceding angular rotations, thus determininga corresponding advance of the sheet 4 which is thus positioned on a newtyping line.

Thus when the peg 66, havingn passed over the constant-radius track 79,descends along the track 80 making the lever 64 turn clockwise, the peg61 is constrained to climb the sloping side of the tooth 60b, againstthe action of the spring 68 until it goes over the crest and falls intothe next tooth, corresponding to return of the peg 66 into the startingposition.

One complete rotation of the cam therefore corresponds to advancing thesheet 4 by six elementary increments, for typing the rows of the matrixformat of the characters of a row of print followed by a row increment,i.e. for the spacing between one row of characters and the next.

The synchronism between rotation of the shaft 55 and the shaft 44,ensured by suitable dimensioning of the diameters of the pulleys 45 and46 and the ratio of the worm gear 56, 57, makes it possible to obtainadvancement by one elementary increment of the sheet 4 for each singlestroke of the slider 10. The time required for rotation of the camthrough the angle of one of the arcs 70 to 76 or of the lobe 77,including the connecting ramps, therefore coincides with the half-periodof oscillation of the slider.

The sensing device 22 comprises a disc 88 carrying two groups of fivediametrically opposed notches 89 and 90, the five nothces of each groupbeing spaced on a circumferential arc having an amplitude less than theangle of rotation of the shaft 44 required to ensure the travel of theslider 10 between one reversing point and the next (180°).

The notches are observed by a photoelectric device 91 of known type,designed to generate, in coincidence with the observation of a notch, asynchronous electrical impulse 92 (FIGS. 6 and 7) to be sent through awire 114 to the electronic control system 30. The photoelectric device91 is positioned relative to the disc 88 as shown in FIG. 1, in such away that, when the slider 10 is in one of the two reversing points,there passes in front of it the mid part of one of the two sectors 89a,90a of the disc 88, free from notches.

Each of the impulses 92 constitutes permission to print for the samematrix point for all the characters to be typed on the row of print.Since the shaft 44 finishes a complete rotation for each stroke of theslider 10, it is clear that each of the two groups of notches 89, 90constitutes permission to print in succession all five points of a rowof matrices for all the characters of a line of print. A row of pointsis typed while the slider carries out a stroke from right to left, andthe next while the same carries out its next stroke from left to right.

The spacing between the notches of each group has to be calculated fromthe laws of movement of the slider, avoiding permission to print for thepoints in which the displacement of the slider relative to its centralposition is close to the maximum value, and e.g. by spacing the notchesof each group on an angle well bellow 180°, it is possible to obtaingood typing results even for a constant spacing between the fivenotches, due to the fact that these limited permissions to print occurat points in which the displacement of the slider from its centralposition as a function of time does not depart very much from a linearlaw.

A notch 93 is cut in the cam 18. The sensing device 27 of known type,for example photoelectric, is designed for noting the passage of thenotch 93 in front of it and to generate, at the same time a synchronouselectric signal 95 (FIG. 6) to be sent, through a wire 131, to thecontrol unit 30. The frequency of this signal is obviously equal to thefrequency of rotation of the cam 18. Since each complete rotation of thecam corresponds to the typing of a row of print on the sheet 4 andadvancement of the latter on to a new printing line, the notch ispositioned on the cam in such a way that every signal generated by thesensor 27 constitutes permission to print a new row of print by thecircuit 30.

We shall now describe the electronic system for controlling the printer,shown according to a block diagram in FIG. 5. A register 100, known initself, is designed to store the codes (e.g. of 6 bits) for the sixteencharacters to be typed on a row of print, sent by the computer to whichthe printer is understood to be, for instance, connected on the channel101. The present block diagram excludes the further connections betweenthe computer and the electronic control system 30, designed to permitactivation and deactivation of the printer by the computer and tosynchronise the flow of data on to the channel 101, since they are knownand do not constitute an object of the present invention.

The register 100 has a number of stages equal to the number ofcharacters of a line of print, each stage being formed by as manybistable elements as there are bits in the codes of the characters (sixin the example considered). The stages are interconnected to form sixparallel shift registers so that information can be shifted from rightto left in bit-parallel, character serial fashion.

The register has an exit channel 104 (of six bits in the exampleconsidered) connected to the outputs of its extreme left stage, whichare, however, connected through a channel 102 and AND gates 105 to theinputs of the extreme right stage of the register.

The channel 101 can be enabled and disabled by means of AND gates 106.The register 100 is controlled by command signals 107 (FIG. 7) conveyedto it by a generator 108 of high frequency shift pulses and is permittedto oscillate in the presence of a signal 92 (FIG. 7) conveyed to it onthe wire 114 through an AND gate 115. Coinciding with each of the shiftpulses, the content of each stage of the register 100 is shifted intothe stage next to the left, and simultaneously there is made to enterinto the extreme right stage the code present on the enabled channel102.

A binary counter 113, known per se, of capacity 35, is designed to countthe synchronising impulses 92 sent to it on the wire 114 through the ANDgate 115 from the sensor 22. Every so often the counter 113 reaches itsmaximum capacity, i.e. its stages assume the combination correspondingto the number 35 and it is designed then to generate a signal of logicallevel "1" on a wire 120.

The outputs of the counter 113 through the channel 126 are connected atthe entrance of a read only memory (ROM) 112 of known type. The outputchannel 104 of the register 100 is also connected at the input to theROM 112. In the ROM 112 there is stored information for determiningwhether or not a selected point of the matrix format is to be printed inthe printing of a selected printable alphanumeric character. Each pointof the matrix format is identified in the ROM by an ordinal number 1 to35 according to the convention shown in FIG. 8.

Corresponding therefore to a particular character code sent to the inputof the ROM 112 through the channel 104 of the register 100, and aparticular point code (ordinal number of a matrix point according to thebinary code of the counter 113) sent as input to the ROM 112 on thechannel 126 of the counter 113, the ROM provides on the output channel138 composed of a single wire, a logical level 1 if it is necessary totype the matrix point selected for formation of the required character,"0" in the contrary case.

A bistable flip-flop 130 is designend to be placed in its "set"condition (with the output Q at the logical level 1) by means of thesignals 95 (FIG. 6) sent to its set input S by the sensor 27, throughthe wire 131. The output Q of the slip-flop 130, when it is at thelogical level 1, activates the AND gates 105. The output Q (which is at1 when Q is at 0) activates, when it is at the logical level 1, the ANDgates 106.

The output channel 138 of the ROM is connected as input to the elementat one end of a shift register 140, known per se, and composed of asmany bistable elements as there are characters in a row of print(sixteen in the example considered here). The register 140 is shifted bythe impulse 107 sent to it by the generator 108 which is connected toit. At each impulse the register 140 is designed to transfer the binaryinformation carried by each bistable element on to the next element andthen make the binary information carried on the channel 138 enter theend bistable element.

The outputs 141 of the bistable elements of the register 140 areconnected as inputs to AND gates 144 whose outputs are connected to thebase electrodes of drive transistors 150 each of which has its collectorconnected to the conductor 35 of a thermoelement 33a and the emitterconnected to a power supply circuit 152. The common conductor 37 of theplate 10 is also connected to the circuit 152. The power supply circuit152 is designed to apply between the conductor 37 and the emitters ofthe transistors 150 a suitable potential difference to make an electriccurrent flow in the thermoelements 33a whose corresponding transistors150 are, as will be seen later, rendered conductive.

The generator 108 also feeds a binary counter 145 of known type having acapacity equal to the number of characters in a row of print (sixteen inthe example considered here). The counter 145 is incremented by thegenerator 108 by one unit for each impulse 107. On reaching its maximumcapacity the counter 145 is designed to return to zero and to generateon and output wire 146 a signal which terminates oscillation of thegenerator 108.

The output wire 146 is connected, through an AND gate 147 opened by thelogical signal present on the wire 120, to the reset terminal R of theflip-flop 130 and to the zeroing terminal Z of the counter 113, so thatthe logical signal generated by the counter 145 on reaching its maximumcapacity causes, when the counter 113 has reached its maximumcomputation, reset of the flip-flop 130 and zeroing of the counter 113.

The output wire 146 is also connected as input to a monostablemultivibrator 149 of known type, whose output Q is connected as input toeach AND gate 144. The monostable multivibrator 149 is designed togenerate in its output Q corresponding to every signal emitted by thecounter 145 on the wire 146, an impulse 155 of logical level 1 having asuitable duration T (FIG. 6). The AND gates are thereby opened totransmit to the base terminals of the drive transistors 150 theseimpulses, only if the corresponding inputs 141 are at the logicallevel 1. The selected transistors 150 are ordered into conduction by theimpulse 155 for the above-stated time T, and there is a circulation ofcurrent in the corresponding thermoelements 33a, leading to heating ofthe latter to a temperature high enough to produce a visible pointimprint on the heat-sensitive recording medium 4.

In describing the method of functioning, the following workingconditions are assumed:

1. Slider in the extreme left position with reference to FIG. 1.

2. printer enabled by the computer to which it is connected, and codesfor the characters to be typed on the next row of print already storedin the register 100.

3. Counters 113, 145 zeroed.

4. Flip-flop 130 in its reset condition (Q = 0).

When the notch 93 passes in front of the sensor 27 there is generated animpulse 95 (FIG. 6) on the wire 131 which forces the flip-flop 130 intoits set position, thus opening the AND gates 105 and 115. The sliderbegins its course to the right and when the first notch of the disc 88passes in front of the sensor 27 an impulse 92 is sent on the wire 114which, through the gate 115, increases by one unit the counter 113.

The same impulse permits oscillation of the generator 108 which sendspulses 107. Each pulse 107 leads to an increase of one unit in thecounter 145, and displacement by one stage to the left of the charactercodes stored in the register 100 and hence sequential presentation inthe farthest left stage (and in the input to the ROM through the channel104) of the character codes of all of the printing positions of a row,and their successive re-entry, through channel 102, into the extremeright stage.

Based on the character code at input and the code of the point positionsent to the ROM respectively from the register 100 and the counter 113(point 1°) the ROM furnishes at the output on the channel 138 a logicallevel 1 if, according to the matrix format, in the position of theselected point it is necessary to type a point for formation of thecharacter in question, 0 in the contrary case.

The register 140 is also commanded by the pulses 107 during each ofwhich it stores in the input end stage the logical level entering andmoves the formation already stored one step on.

When the counter 145 has reached its maximum count the register 100 hasthe same initial configuration, the codes having returned to thestarting position after having passed through all the stages. Theregister 140 carries in its bistable elements the information type ornot type for each of the thermoelements 33a, and the corresponding ANDgates 144 are allowed to open only if the logical level of thecorresponding output 141 of the register 140 is 1.

As stated, the counter 145, having reached its maximum capacity,generates an impulse which stops the generator 108, zeroes the counteritself, and orders into conduction the permitted transistors, throughthe monostable multivibrator 149 for a definite time T, but in any caseless than the time intervening between the passage of one notch and thenext in front of the sensor 22, thus inducing the passage of current inthe permitted thermoelements 33a and typing, in each printing positionof the typing line of sheet 4, a point whose breadth in the direction ofmotion depends on the excitation time T and the velocity of the slider10.

It is to be noted that the whole loading of the register 140 takes placeat "electronic" velocity and therefore in a time which is extremelyshort relative to the mechanical movement of the slider. Evenconsidering for the slider very high mechanical velocities, theabove-stated time is only a minimal portion of the time interveningbetween the passage of one notch and the next of the disc 88 in front ofthe sensor 22. The same procedure is repeated for the passage of thenext four notches in front of the sensor 22, and each time there is anincrement of one unit in the counter 113 and the typing of one point ofthe first row of the matrix format for all characters of one row ofprint.

After the passage of the first group of five notches, the slider is atthe end of its run to the right and gets ready to reverse its motion,and the roller 7 and hence the sheet 4 are advanced by an elementarystep by the cam 18 and the pawl system 17. The functioning then proceedsas in the typing of the first row, and so also for the typing of allseven rows.

At the end of typing the seventh row and hence of the points 35, thecounter 145 having reached its maximum capacity generates, as alreadyseen, a reset signal through flip-flop 130, which bringing its output Qto zero, closes the AND gate 115 and inhibits computation by the counter113, which is zeroed, and the starting of the generator 108.

At the end of typing the seventh row of the matrix format, the sliderwill be in the extreme right position and will carry out the next runtowards the left without any typing operation. During this return run,the peg 66 engages with the lobe 77 of the cam 18 and makes the papercarry out the spacing necessary for positioning it on a new printingline, and the computer sends on channel 101 the character codes fortyping on a new printing line, or, for example, the order to stop theprinter.

At the end of each run the passage of the notch 93 in front of thesensor 27 will produce the setting of the flip-flop 130 and thebeginning of the operation of typing for a new row of print.

It is to be understood that various modifications are possible withinthe scope of the claims. For example, a variant may be that of halvingthe number of thermoelements 33a present in the head, doubling thespacing between them and the amplitude of oscillation of the slider, andin such case with suitable modifications to the control circuit 30, itis possible to obtain during each stroke of the slider, from a singleelement, one of the rows of the matrix format of two adjacent charactersof the row of print.

What I claim is:
 1. An electrothermal printing unit for the non-impactprinting of characters in accordance with a character matrix of dots ona thermosensitive recording medium comprising:means for supporting saidmedium and for moving the medium incrementally along a first direction,a print assembly including a slidable member and a thermal print headmounted on said slidable member, said head having a plurality ofenergizable thermal printing elements arranged in a single row in astraight line of print perpendicular to said first direction, the spacebetween two adjacent printing elements being equal, each of saidprinting elements being able to print a dot on the recording medium whenenergized, said slidable member being mounted for movement along saidline of print with said elements in contact with the recording medium,means for transporting said slidable member in an oscillating motionthrough a plurality of printing strokes along said line of print, theamplitude of each stroke being at least equal to the distance betweentwo adjacent elements, means for storing codes identifying a line ofcharacters to be printed, means, responsive to said storing means, forselectively energizing said printing elements to print dots on therecording medium in the line of characters configuration during eachprinting stroke, each of said printing elements being operational toprint a portion of a line of dots simultaneously with the printing ofother portions of the line by the other printing elements, means coupledto said slidable member for sensing the position of said slidable memberas it moves along said line of print and for enabling said selectiveenergizing means to print dots on said recording medium only when saidslidable member is in predetermined positions in its stroke across themedium, said means for moving the recording medium having means foradvancing said medium along said first direction by a predeterminedamount after each printing stroke across the medium for positioning saidmedium to receive another line of dots.
 2. A printing unit according toclaim 1, wherein the number of printing elements is equal to the numberof characters printable in a line, said energizing means effecting anumber of selective energizations of the printing elements during theprinting of a line of characters equal to the number of dots of saidmatrix.
 3. A printing unit according to claim 1, wherein said charactermatrix has columns and rows and said energizing means selectivelyenergizes the printing elements during strokes of the head along saidline of print in both directions, the said plurality of printing strokesbeing a number of consecutive strokes equal to the number of rows of thematrix.
 4. An electrothermal printing unit for the non-impact printingof lines of characters, each line having a maximum predetermined numberof character positions, each character being printed according to acharacter-matrix of points arranged in a plurality of rows,comprising:means for supporting a heat sensitive record medium formoving the medium incrementally along a first direction, each incrementpositioning the medium to receive the printing of a different row ofpoints of said character-matrix for all the characters of a line, aprint head carrying a number of thermally activable printing elementsequal to said maximum predetermined number and disposed in a single rowand spaced apart at least the distance between two adjacent characterpositions, means mounting said head with the printing elements incontact with the recording medium and with said single row parallel to asecond direction perpendicularr to said first direction and defining theline of print direction, means for reciprocating the head in said seconddirection during the printing of a line of characters, through aplurality of printing strokes, in synchronism with the movement of therecording medium, the amplitude of each stroke being at least equal tothe distance between adjacent elements, means for signalling theposition of the head during each printing stroke including a support,movable in synchronism with the head, carrying a plurality of markersthereon and a sensor device for sensing said markers and for generatingcorresponding electrical signals, means for storing codes deignating aline of characters to be printed, and control means responsive to saidelectrical signals and said storing means for selectively andsimultaneously activating the printing elements a number of times equalto the number of points contained in a row of said matrix for eachprinting stroke of the head, whereby for each stroke each elementselectively prints all the points of a row of the matrix of a characterto be printed in the line.
 5. A printing unit according to claim 4,wherein each of said elements has a thermo-activable printing surfacesubstantially rectangular in shape, with the greater dimensionperpendicular to the line of print direction and substantially equal tothe height of the points of the matrix, the control means activating theelements for a sufficient time to form, with the movement of the head,substantially square matrix points.
 6. A printing unit according toclaim 4, in which the supporting means comprises a platen, a programmingcam for rotating the platen and having an active profile with a seriesof first steps with constant rise and a second step with rise greaterthan the first steps, a cam-following element cooperating with theactive profile to turn the platen incrementally to advance the recordingmedium through increments corresponding to the distance between twoadjacent rows of the matrix, when it cooperates with the first steps,and to effect a rotation of the platen and advancement of the recordingmedium corresponding to an interline space between two rows ofcharacters, when it cooperates with the second step.
 7. A printing unitaccording to claim 6, wherein the reciprocating means comprise a speedmotor and a crankshaft actuated by the motor, the head beingmechanically connected to the crankshaft, the motor also actuating theprogramming cam.